1. Field of the Invention
The present invention relates to a liquid crystal display device and a process for producing the same. In particular, the present invention relates to a liquid crystal display device having excellent mechanical strength as well as sealing property without any poor display caused by a sealing material, and a process for producing the same.
2. Description of the Related Art
A liquid crystal display device comprises a pair of insulating substrates (typically, glass substrates) 101 and 102, and a liquid crystal layer 110 as a display medium which is disposed between the substrates, as illustrated in FIG. 36. On the glass substrate 101, an active element (typically, thin film transistor (TFT)) which controls the electro-optical characteristics of the liquid crystal, and a scanning line and a signal line which give a gate signal and a source signal, respectively, to the active element are provided. This substrate 101 is designated as a TFT substrate (or an active matrix substrate). On the glass substrate 102, color layers 106, 107 and 108 which constitute color filters, and a black matrix 105 which is a light-shading layer are formed. This substrate 102 is designated as a color filter substrate.
Ordinarily, the TFT substrate 101 and the color filter substrate 102 are adhered together by a sealing material 103 to form a cell. In adhering these substrates, the TFT substrate 101 and the color filter substrate 102 are adhered together by using an intra-sealing spacer 304 and a liquid crystal layer spacer 303 at a certain gap (a cell gap). The cell gap is approximately 4 .mu.m to 6 .mu.m and its variation is .+-.10% in the conventional TN-type liquid crystal display device.
The intra-sealing spacer 304 is included in the sealing material 103. Usually, the diameter of the intra-sealing spacer is about 5 .mu.m. The sealing materials that can be used include thermosetting resins and ultraviolet curable resins. The sealing material is coated on either the TFT substrate or the color filter substrate in the prescribed pattern by a screen printing method, a letterpress printing method or a dispenser coating method. According to each coating method, the viscosity of the sealing material is adjusted at a level optimal for the coating. The viscosity is usually adjusted by the addition of a filler (for example, silicon oxide or alumina having a particle size of 1 .mu.m to 3 .mu.m). The liquid crystal layer spacer 303 is sprinkled in a portion of the substrates on which the sealing material is not coated. Usually, the diameter of the liquid crystal layer spacer is also about 5 .mu.m. The liquid crystal layer spacer is sprinkled in an amount of about 100 pieces/mm.sup.2.
In adhering the substrates, the TFT substrate and the color filter substrate are aligned, and then a sufficient load applied so that the prescribed cell gap is retained, and heated or irradiated with ultraviolet ray at that state. The heating and ultraviolet ray irradiation conditions may be varied depending upon the hardening characteristics of the sealing material, and the load may be varied depending upon the sizes of the substrates, and the area and viscosity of the sealing material, and the like. Then, a liquid crystal material is injected into the cell from an inlet formed in a portion of the sealing material 103 by, for example, a vacuum injection method, and further the inlet is sealed by, for example, an ultraviolet curable resin. Thus, the liquid crystal display device is completed.
Such a liquid crystal display device provides display information on the screen by selecting a display pixel arranged in a matrix shape. For example, an active matrix-type liquid crystal display device (a type using a switching element (an active element) such as TFT for each pixel electrode as a selective procedure of the display pixel) is capable of providing a high contrast display, and has been widely used for liquid crystal televisions, notebook-type personal computers, and the like. The switching element of the active matrix-type liquid crystal display device has the function of turning on and off a signal voltage applied between the pixel electrode and a transparent electrode (counter electrode) formed opposite to said pixel electrode through the liquid crystal layer, and displays an image information by the optical variation of the liquid crystal layer caused by the potential difference between the pixel electrode and the counter electrode.
A gate bus line which is a scanning line operating the pixel electrode and the switching element, and a source bus line which is a signal line for applying a signal voltage are usually formed through, for example, a silicon nitride insulating film of several hundreds to one thousand and several hundreds angstroms, and formed at a gap of several microns to ten and several microns.
A liquid crystal display device comprising an interlayer insulating film made from for example silicon nitride and having a thickness of several thousands angstroms which is formed on the gate bus line and the source bus line, and pixel electrodes further formed thereon has been proposed (e.g., Japanese Laid-open Publication No. 58-172685). According to this liquid crystal display device, since it is possible to form a pixel electrode also on each bus line, the area of the pixel electrodes can be enlarged to provide the increased light transmittance (i.e., aperture ratio) of the liquid crystal display device.
The following illustrates one example of the structure of an active matrix substrate used for this liquid crystal display device. FIGS. 37 and 38 are a schematic plan view and a schematic cross-sectional view of the active matrix substrate, respectively. The active matrix substrate comprises a gate electrode 609, a gate insulating film 610, a semiconductor layer 611, an n.sup.+ -Si layer 612 which constitutes a source and drain electrode, a metal layer 613 which constitutes a source signal line, an interlayer insulating film 607, and a transparent conductive layer 603 which constitutes a pixel electrode, all of which are formed in this order on a transparent insulating substrate 608. The pixel electrode is electrically connected to the drain electrode of the TFT through a contact hole 614 penetrating the interlayer insulating film 607. Since the interlayer insulating film is formed between the pixel electrode and the scanning and signal lines in the active matrix substrate illustrated in the figures, it is possible to form the pixel electrode as overlapping with the signal line.
On the effective display portion of the finished active matrix substrate, an alignment film made from a polyimide or the like is formed to provide an alignment function by a treatment such as rubbing, UV irradiation, and the like. Also, a transparent counter common electrode is formed using ITO (indium tin oxide) or the like on the counter substrate, and thereafter its effective display portion is subjected to the same treatment. A sealing material is coated on the periphery portion of the panel by a printing procedure or the like in such a manner that the panel is surrounded by the sealing material. An inlet is formed in a part of the sealing material. Moreover, a conductive material is attached to the signal input terminal for the counter electrode located on the active matrix substrate. Then, a spacer is sprinkled so as to provide a uniform cell gap of the liquid crystal layer, the liquid crystal layer and the counter electrode are aligned, and the sealing material is heated and cured. Thereafter, a liquid crystal is injected from the liquid crystal inlet which is then closed with a sealing material to complete the glass portion of the active matrix-type liquid crystal display device. Since a flat surface is obtained according to the liquid crystal display device using such an active matrix substrate, there is a benefit that the orientation disturbance of the liquid crystal molecules inside the display region can be prevented.
A liquid crystal display device using a photosensitive transparent acrylic resin as the interlayer insulating film has also been proposed. The acrylic resin has the following benefits: (1) providing a high transmittance in the prescribed visible light region; and (2) permitting the reduction of a capacity between each line and the pixel electrode due to easy control of the film thickness to result in the reduction of the cross talk, and the like.
However, the above-described liquid crystal display devices suffer from a problem as having poor display caused by the sealing material.
Since the sealing material includes a filler and a spacer, it has the following problems when applying a load in adhering the substrates. The conditions under which the load is imposed (for example, a period until the load reaches the prescribed level (a load imposing rate), a cell gap, and the like) may be varied depending upon the applications (the types) of the intended liquid crystal display device. Therefore, for example, when applying a large load such that a sealing material using an ultraviolet curable resin is compressed up to a level less than two-fold of the filler diameter, a separation phenomenon is observed in which a low viscous resin component contained in the sealing material is separated from the additives (e.g., the filler, the spacer) and flows out. This separation phenomenon may occur in the case of using not only an ultraviolet curable resin, but a thermosetting resin. Since the thermosetting resin is once softened with heating and thereafter cured, a rapid viscosity reduction of the resin material during the softening may cause a separation between the resin component and the additives. Although the separation phenomenon itself does not affect the characteristics of the liquid crystal display device, it may cause poor display in the case where the separated resin component flows out into the display portion. Ordinarily, the separated resin component flows through the concave portion of the uneven portion proximate to the sealing material (i.e., using the concave portion as a gutter) into the display portion. These uneven portions include an unevenness of the lines formed on the surface of the TFT substrate (having a difference in level of 3000 angstroms to 5000 angstroms), an unevenness between the black matrix of the color filter and the color layers (difference in level of about 15000 angstroms), and the like.
For the purpose of improving these problems, an attempt has been made to prevent the resin from flowing out using the concave portion as a gutter by flattening the surface of each substrate with an insulating film, and the like. However, because the resin flows out randomly, the poor display is not eliminated.
Recently, an attempt at changing the BM material of the color filter from a metal material (such as Cr or Al) to a resin material has been made for the purpose of reducing the cost. The film thickness of the metal material BM is about 2000 .ANG.. However, when a resin material is used as the BM material, it is necessary that the film thickness of the resin material BM be about 1 .mu.m or more so as to provide a shading property equivalent to that of a metal material. This thickness is about the same as that of the color filter (about 1.3 .mu.m). A liquid crystal display device using a metal BM has a different cell gap between the display portion and the sealing material portion, and the diameter of the spacer sprinkled on the display portion, dLC and a cell gap of the sealing material portion (i.e., the thickness of the sealing material resin), dS have always such a relationship, dLC&lt;dS. Therefore, the sealing material is not so compressed during adhering the substrates. On the other hand, when a resin BM is used, dLC.apprxeq.dS. In general, dLC is about 5 .mu.m, and dS is about 6 .mu.m which is almost equivalent to dLC. Thus, the compression of the sealing material will be increased by 20% during adhering the substrates. As a result, a low viscous resin component contained in the sealing material is liable to be separated from the additives.
It is theoretically possible to solve the above-described problems by optimizing the load and heating conditions. However, because various types of liquid crystal display devices are usually manufactured using the same manufacturing apparatus, an enormous labor is required for the determination of the load and heating conditions especially at the time of introducing a novel type of manufacturing apparatus. Therefore, this attempt is not practical.
The following further illustrates the problems of an active matrix-type liquid crystal display device.
According to an active matrix-type liquid crystal display device, when a sealing material is cured in adhering the active matrix substrate to the counter substrate in, a gushing phenomenon of a solvent contained in the sealing resin may occur. That is, the bumping of the sealing material may take place with the heat due to the thermal distribution and the variation of the blended components before the sealing material is completely cured. Thus, a solvent or a component (e.g., a filler) for adjusting the viscosities of the sealing material overflows from a region in which the sealing material is originally formed. The gushing phenomenon is largely associated with the cell gap of the sealing portion, as illustrated in detail. In adhering the substrates together and curing the sealing material, the heat and pressure are simultaneously applied and the desirable cell gap is controlled by a spacer included in the sealing material and a spacer sprinkled on the substrates prior to the adhering. When a substrate gap (a cell gap) is narrow in the sealing portion, a great pressure is concentrated together with heat on the sealing material, which mainly contributes to the gushing, and in an extreme case, the gushed sealing material reaches the effective display area. Even when it does not reach the display area, there is also a reliability problem that the sealing material has a different formulation from the desirable formulation due to the gushing of the sealing material components, and thus remains uncured inside the cell even after the completion of the panel and is oozed out into the display region during using to cause poor display. For that reason, it is thought that the interlayer insulating film (which is the thickest component of the active matrix substrate) under the sealing material is removed. However, when the interlayer insulating film in this portion is completely removed by patterning, another problem arises that it is difficult to ensure the uniformity of a substrate gap (a cell gap) in the entire panel. The interlayer insulating film is formed by a spin coating method, and the like. Since the required thickness of the interlayer insulating film is generally 3 .mu.m or more, it is extremely difficult to provide this film having a uniform film thickness in the entire portion of a large substrate. Therefore, when the interlayer insulating film is not formed under the lower portion of the sealing material and it is formed on the display region, the cell gap of the display portion is varied in accordance with the variation of the film thickness of the interlayer insulating film.
Then, there has been proposed a structure that the film thickness of a part of the interlayer insulating film under the sealing material is made thinner instead of removing it completely. FIGS. 39 and 40 are a schematic plan view of such a substrate and a cross-sectional view taken along with the XL--XL line of this substrate, respectively. According to this structure, since a part of the interlayer insulating film under the sealing material has a film thickness thinner than the other part by the prescribed amount, even when the film thickness of the interlayer insulating film is varied, the film thickness of a part of the interlayer insulating film under the sealing material has the same variation, and therefore the film thickness difference between the thinner film thickness portion 604 and the other portion 607 remains at the same level. The film thickness is controlled by the exposure and development periods when the interlayer insulating film is made from a photosensitive material, and the etching period when it is made from a non-photosensitive material.
When a thinner film thickness portion is formed in the prescribed portion of the interlayer insulating film, this portion may have poorer chemical resistance than the other portion. For example, when the thinner film thickness portion is formed by an exposure and development procedure using a positive-type photosensitive resin, it is exposed to a light for a shortened suitable exposure period than the case where the film is completely removed by patterning which requires a sufficient exposure of the light, the exposure stopped at the time when its crosslinking structure is partially decomposed, and then developed. Therefore, this partially exposed portion may have poorer chemical stability compared to the non-exposed portion. Also, when it is formed by a photolithography and etching procedure using a non-photosensitive resin, the thinner film thickness portion may be structurally unstable compared to the other portion because it is exposed to an etchant, and the like. When a transparent conductive film which constitutes a pixel electrode is formed on the interlayer insulating film thus formed, defects such as film lifting and peeling may occur on the interlayer insulating film because the thinner film thickness portion of the interlayer insulating film comes into contact with the developer and the etchant.
Moreover, the adhesion of both substrates of a liquid crystal display device having an interlayer insulating film is made by a sealing material through the interlayer insulating film (for example, an acrylic resin film) on the TFT substrate. Since the sealing material has an insufficient adhesive strength to the interlayer insulating film, the resulting liquid crystal display device has insufficient mechanical strength as well as sealing property.
As described above, a liquid crystal display device having excellent mechanical strength as well as sealing property without any poor display caused by the sealing material, and a convenient process for producing the same are desirable.